docs(whp): add backend-switching how-to + Optimized Webserver (OLS) add-on page

- how-to/switching-site-backend: change a site's container type between standard
  PHP/FPM and the premium LiteSpeed/OLS backend; required steps to move off
  premium before cancelling the add-on (panel blocks the disable otherwise).
- add-ons/optimized-webserver: what the OLS/LSCache add-on is, advantages, when
  NOT to use it, how to enable/use, and cancellation requirements.
- add-ons/overview: list the new add-on.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/content/docs/whp/add-ons/overview.mdx

@@ -20,6 +20,7 @@ Add-ons are **optional extras** you can layer onto your base hosting plan. Each
 - **[Archival email](/whp/add-ons/archival-email/)** — long-term, searchable retention of your mail outside the live mailbox.
 - **[Resource upgrades](/whp/add-ons/resource-upgrades/)** — extra CPU, RAM, or storage without changing plans.
 - **[Email upgrades](/whp/add-ons/email-upgrades/)** — extra mailboxes or larger per-mailbox storage caps.
+- **[Optimized Webserver](/whp/add-ons/optimized-webserver/)** — run your sites on OpenLiteSpeed with server-level LSCache for dramatically faster page delivery.
 
 ## How billing works